1. Field of the Invention
The present invention relates to a body frame structure of a vehicle, particularly a motor vehicle.
2. Description of the Related Art
In a conventional motor vehicle body frame, for example, a floor frame structure extending in the longitudinal direction beneath a floor panel of a vehicle body and being fixed thereto as shown in FIG. 9 is formed by integrally joining, by welding and the like, the rear end of a front floor frame of a front half vehicle body and the front end of a rear floor frame of a rear half vehicle body, the front floor frame being designed to have a high strength, and the rear floor frame being designed to have a strength lower than that of the front floor frame. Further, in some cases, a support member is fixed to the rear floor frame having the lower strength, in the joined section in order to support a component such as a fuel tank.
However, when a motor vehicle employs the floor frame structure described above, as shown in the graph (broken line) of FIG. 8 and the perspective view of a floor frame of FIG. 9, welding of the support member generates in the joined section a local area having a cross sectional strength (medium strength area) higher than the other areas of the rear floor frame having the lower strength, so that the cross sectional strength largely changes in the longitudinal direction of the vehicle. In other words, in the joined section between the front floor frame and the rear floor frame, the cross sectional strength changes rapidly, not gradually and continuously in the longitudinal direction. As a result, when an excessively large external force acts on the floor frame in the longitudinal direction, there is a problem that the floor frame buckles at the low strength area of the joined section.
The present invention has been achieved in view of the above-mentioned circumstances to provide a novel vehicle body frame structure that can prevent a frame from buckling by making the cross sectional strength gradually decrease from a first frame member (front floor frame) to a second frame member (rear floor frame) in a joined section where the first frame member and the second frame member are joined together.
In order to accomplish this, in accordance with a first aspect of the present invention, a vehicle body frame structure has a first frame member having a high strength, and a second frame member having a strength lower than that of the first frame member, the first frame member and the second frame member being joined integrally in line with each other, the second frame member having a locally reinforced region in a joined section by fixation of a support member for supporting a component, wherein a joint stiffener is fixed to the second frame member, the stiffener functioning so that the strength of the joined section gradually decreases from the first frame member to the second frame member. Furthermore, in accordance with a second aspect of the present invention, there is a vehicle body frame structure according to the first aspect wherein the strength of the locally reinforced region of the second frame member is lower than that of the first frame member.
In accordance with each of the aspects of the present invention, even when the reinforced region is locally present, due to the support member being fixed for supporting another component, in the joined section between the first frame member having the high strength and the second frame member having the low strength, the strength of the joined section can be decreased gradually from the first member to the second member, thereby preventing the joined section from buckling.
The above-mentioned object, other objects, characteristics and advantages of the present invention will become apparent from an explanation of a preferred embodiment that will be described in detail below by reference to the attached drawings.